The invention relates to an electronic switching device. The electronic switching device includes a first semiconductor component. The first semiconductor component has a first cathode connection, a first anode connection, and a first grid connection. The electronic switching device further includes a second semiconductor component. The second semiconductor component has a second cathode connection, a second anode connection and a second grid connection. The first anode connection and the second cathode connection are electrically short-circuited.
Such an electronic switching device is known from WO 97/34322 A1 corresponding to commonly-owned U.S. Pat. No. 6,157,049, and from U.S. Pat. No. 5,396,085. The respectively disclosed electronic switching device also includes an electrically conductive connection between the first cathode connection and the second grid connection. This interconnection of two semiconductor components is also referred to as a cascode circuit. The electronic switching device is used for switching high electric current, and is also configured for a high reverse voltage. The first semiconductor component is made of silicon (Si) and, owing to the high charge carrier mobility in the silicon, ensures a high switching speed. The second semiconductor component is composed of a semiconductor material having a breakdown field strength of more than 106 V/cm, in particular of silicon carbide (SiC), and ensures a high reverse voltage.
In contrast, an electronic switching device that is produced using only silicon, for example a voltage-controlled Si-MOSFET (Metal Oxide Semiconductor Field Effect Transistor), has steady-state losses in the switched-on state which rise with the reverse voltage which has to be coped with by the Si-MOSFET in the switched-off state. In silicon, the steady-state power loss of a power MOSFET configured for a reverse voltage of more than 600 V is excessive for a forward current of more than 5 A. For this reason, Si-MOSFETs are no longer used, despite the high switching speed, for applications with a reverse voltage and a forward current in the said order of magnitude.
According to WO 97/34322 A1, the first semiconductor component is composed of Si and, overall, for a given polarity of the operating voltage, the electronic switching device also can be switched between a switched-on state and a switched-off state using a control voltage that is present at the first grid connection. When the electronic switching device is switched off, a depletion zone (zone with a reduced number of charge carriers and thus a high electrical resistance; space-charge zone) of at least one p-n junction constricts at least one channel region of the semiconductor component, which is composed of SiC. The majority of the operating voltage which is to be switched off and is applied between the first cathode connection and the second anode connection is dropped across this depletion zone. Owing to the high breakdown field strength of the silicon carbide that is used, the p-n junction, in particular its depletion zone, can withstand a considerably greater reverse voltage than a p-n junction formed from silicon and with the same charge carrier concentrations and dimensions. Because the majority of the reverse voltage is dropped within the second semiconductor component, the first semiconductor component thus need be configured only for the remaining part of the reverse voltage. This results in considerably reduced power losses in the first semiconductor component, which is composed of silicon, when switched on.
When switched on, the depletion zone of the p-n junction in the second semiconductor component is flooded with charge carriers, and the channel region is opened. An electric current now can flow through the channel region. The total power loss in the electronic switching device then includes the losses in the first and second semiconductor component. These total losses are now considerably less than those with a pure silicon semiconductor component configured for the same reverse voltage.
Integration of the two semiconductor components to form a hybrid semiconductor structure is also known from WO 97/34322 A1. The metallization, which is applied to the entire area of the surface of the second semiconductor component composed of SiC, for the second cathode connection is in this case at the same time used as the metallization for the first anode connection of the first semiconductor component, which is composed of Si.
A similar electronic switching device have a cascode circuit formed by a first semiconductor component composed of Si and a second semiconductor component composed of SiC is known from U.S. Pat. No. 5,396,085. One difference, however, is the use of a composite substrate, which contains not only an area composed of silicon but also an area composed of silicon carbide. The two semiconductor components are each produced in one of these areas of the composite substrate.
Furthermore, a cascode circuit formed from a normally-off MOSFET composed of silicon and an SIT (Stated Induction Transistor) composed of a composite semiconductor, for example of gallium arsenide (GaAs) or indium phosphide (InP) is also known from JP 61-161015 A1. This electronic device is in this case primarily used for extremely fast switching for a radio-frequency application.
In general, in the described cascode circuit, the forward resistance of the first semiconductor component has a negative-feedback effect to the second grid connection of the second semiconductor component. As the current through the electronic switching device increases, the negative bias voltage on the second grid connection also rises in comparison to the second cathode connection. The depletion zone of the p-n junction, which is located between the two connections, is thus further enlarged into the channel region intended for the current flow. Thus, as the current through the electronic switching device rises, the forward resistance of the second semiconductor component is increased, however.
In order to at least partially overcome this effect, the electrical switching device disclosed in German patent DE 34 07 975 C2, corresponding to U.S. Pat. No. 4,523,111, provides for a p-n junction in the second semiconductor component to be appropriately biased. This p-n junction is located between the second grid connection and the second cathode connection within the second semiconductor component, which is in the form of a junction field-effect transistor (JFET). The bias voltage is in this case dimensioned such that the p-n junction, and thus also the JFET overall, are in a bipolar conduction state. The bias voltage is thus greater than the diffusion voltage of this p-n junction. For silicon, the diffusion voltage is in the order of magnitude of 0.6 to 0.7 V. For bipolar operation of the p-n junction, the second semiconductor component is now no longer driven without any power consumption. A current flows via the p-n junction. Owing to this current flow, the second grid connection needs to be configured to be more stable and, in particular, also larger, as a result of which space is lost for the actual active area of the second semiconductor component. This reduces the current switching capacity of the electrical switching device. The current flow at the second grid connection furthermore leads to a capacitance of the p-n junction first of all having to be charged up or having its charge reversed when a switching process is initiated. The achievable switching speed thus also falls.
It is accordingly an object of the invention to provide an electronic switching device having at least two semiconductor components that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has a low forward resistance and, at the same time, a good current switching capacity and a high switching speed.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an electronic switching device. The electronic switching device includes a first semiconductor component and a second semiconductor component. The first semiconductor component has a first cathode connection, a first anode connection, and a first grid connection. The first grid connection receives a control voltage. The second semiconductor component has a second cathode connection, a second anode connection, a second grid connection, and a p-n junction located between the second grid connection and the second cathode connection. The p-n junction has a diffusion voltage. The second grid connection also receives a part of the control voltage. The part of the control voltage is applied to the second grid connection and produces a grid-cathode voltage of the second semiconductor component. The grid-cathode voltage of the second semiconductor component always is kept less than the diffusion voltage of the p-n junction by manipulating the part of the control voltage received by the second grid connection. The first anode connection and the second cathode connection are electrically short-circuited.
In the electronic switching device, the invention provides that a control voltage that can be applied to the first grid connection is also partially present at the second grid connection. In which case, the part of the control voltage which is applied to the second grid connection is dimensioned such that a grid-cathode voltage of the second semiconductor component is always less than a diffusion voltage of a p-n junction that is located between the second grid connection and the second cathode connection within the second semiconductor component.
The invention is in this case based on the knowledge that the advantageous effect of the first and second semiconductor components, with their respective specific advantages, also can be achieved if the second grid connection is not short-circuited to the first cathode connection. Disconnection of this electrical connection provided in the prior art also offers the advantage that the negative-feedback effect of the forward resistance of the first semiconductor component to the second semiconductor component can be avoided with a consequent increase in the forward resistance of the second semiconductor component. This decoupling means that the forward resistance of the second semiconductor component in the rated current region is essentially independent of any electric current flowing via the electronic switching device. The partial coupling of the control voltage that is present at the first grid connection to the second grid connection means, specifically, that a depletion zone which is caused by a diffusion voltage in a channel region in the second semiconductor component is considerably reduced, thus resulting in the reduced forward resistance. As the forward resistance of the second semiconductor component falls, the total forward resistance of the electronic switching device is also reduced.
As a result of the fact that the part of the control voltage which is applied to the grid connection results in a grid-cathode voltage in the second semiconductor component which is always less than the diffusion voltage, means that the p-n junction is also not switched to the bipolar state. That is to say, the p-n junction is not switched to the conductive state. No significant current flow takes place at the second grid connection. This results in a very low forward resistance with the current switching capacity remaining equally good, and the switching speed likewise being equally fast. The drive advantageously takes place, as before, without any power, so that the dimensioning of the second grid connection can remain equally small. Furthermore, no significant bipolar injection into the p-n junction occurs.
The measure of applying a part of the control voltage that is present at the first grid connection to the second grid connection as well means that the reverse-voltage capacity of the electronic switching device is not adversely affected in any way. Since, in particular, a control voltage of 0 V is required at the first grid connection for switching to the switched-off state, the second grid connection is thus also virtually at the same potential as the first cathode connection. However, this means that relationships that are comparable to those in the prior art exist in the switched-off state. The positive control voltage required for switching to the switched-on state at the same time, however, owing to the partial coupling to the second grid connection also results in an improved current carrying capacity in the channel region for the current flow in the second semiconductor component. In the switched-off state, this channel region is constricted, as in the prior art, by a depletion zone, with the overall depletion zone bearing a large proportion of the reverse voltage that is present across the electronic switching device.
In one particularly advantageous embodiment, the part of the control voltage that is applied to the grid connection leads to a grid-cathode voltage with a value of at most two-thirds of the diffusion voltage of the p-n junction. This p-n junction is located between the second grid connection and the second cathode connection within the second semiconductor component. This additional safety margin for the diffusion voltage further reduces the current flow, which is already low in any case, via the p-n junction.
The electrical partial coupling of the potentials at the two grid connections can advantageously be achieved using a first electrical coupling resistance connected between the two grid connections. The dimensioning of this first coupling resistance allows the proportion of the control voltage that is present at the first grid connection and is intended to be dropped at the second grid connection to be set well. This setting can be made even more exactly if, as in a further advantageous embodiment, the proportion of the control voltage that is dropped at the second grid connection is tapped off via a voltage divider that is connected between the first grid connection and the first cathode connection. This voltage divider includes a first and a second coupling resistance.
In order to improve the dynamic response, one advantageous refinement provides for a first diode to be connected in parallel with the first coupling resistance, and a second diode to be connected in parallel with the second coupling resistance. In consequence, when switching between the switched-off state and the switched-on state, the charging or discharging process of a capacitor between the second grid connection and the second cathode connection is hastened.
In one preferred embodiment, the first coupling resistance or the voltage divider is purely resistive.
However, another type of impedance, for example a capacitive impedance or any complex impedance, is also possible.
In one embodiment, the first semiconductor component is advantageously produced using the semiconductor material silicon. Owing to its high charge carrier mobility, silicon allows a very high switching speed.
A further advantageous refinement provides for the first semiconductor component to be in the form of a normally-off MOSFET. In this case, the first semiconductor component can be switched to the switched-on state via a voltage that is present at the first grid connection. The first grid connection in this embodiment corresponds to the gate electrode of the MOSFET. A normally-off MOSFET with an n-conductive channel is preferable. This allows the MOSFET to be switched from the switched-off state to the switched-on state via a positive control voltage.
In another embodiment, the second semiconductor component is advantageously produced using a semiconductor material having a breakdown field strength of at least 106 V/cm. This results in the high reverse voltage capacity required by the second semiconductor component in the switched-off state. Suitable semiconductor materials are diamond, aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) and, in particular, silicon carbide (SiC). For the latter, the polytypes 3C-, 4H-, 6H-, and 15R-SiC are particularly suitable.
A further advantageous embodiment includes a second semiconductor component that is in the form of a normally-on field-effect transistor. This allows the current flowing via the electronic switching device to be controlled very easily and quickly just by a switching operation in the first semiconductor component.
The normally-on field-effect transistor is preferably in the form of a junction field effect transistor (JFET). In this type of transistor, the current flow through a depletion layer that can be influenced externally, in particular a depletion zone for example of a p-n junction, is controlled. The physical extent of this depletion zone then governs the magnitude of the current flowing. Depending on the voltage that is present at the second grid connection, the depletion zone then opens or constricts a channel region with a greater or lesser width for the current flow.
The following refinements are preferred embodiments of the junction field-effect transistor (JFET).
In a first refinement, the JFET includes an n-conductive first semiconductor region. On one surface of this first semiconductor region there is a contact region that is likewise n-conductive within the first semiconductor region. This contact region can be doped to the same extent or else to a greater extent than the rest of the first semiconductor region. It is made contact with electrically, in particular resistively, via the second cathode connection. An area of the surface of the first semiconductor region located outside the contact region is made contact with electrically via the second grid connection. This contact may not only be resistive but may also be in the form of a Schottky contact. It is possible for a number of contact regions as well as a number of areas located outside these contact regions to be provided, which are then each made contact with by the second cathode connection or the second grid connection. The first semiconductor region is made contact with electrically, in particular resistively, via the second anode connection, on a side facing away from that surface. Because the current flows vertically between the second cathode connection and the second anode connection, that is to say at right angles to the surface, through the second semiconductor component, it is also referred to as a vertical JFET.
In a second refinement, a p-conductive second semiconductor region is located on the surface within the first semiconductor region. This second semiconductor region is so highly doped that the second grid connection forms a resistive contact. It is also possible for the JFET to contain a number of second semiconductor regions. A p-n junction with a depletion zone is formed between the n-conductive first semiconductor region and, possibly, also the likewise n-conductive contact region on the one hand, and the p-conductive second semiconductor region on the other hand. This depletion zone forms the boundary layer in the JFET, which can be controlled via the second grid connection. A channel running within the first channel region is constricted or even completely covered by this depletion zone by applying an appropriate voltage to the second grid connection.
In particular, a third refinement is also advantageous, which includes a p-conductive island region buried within the first semiconductor region. A number of these buried island regions also optionally may be provided. The buried p-conductive island region is, in this case, disposed in particular such that, in a projection at right angles to the surface, the projection of the contact region is located completely within the projection of the buried island region. This results in a lateral n-conductive channel region: that is, a region running parallel to the surface, within the first semiconductor region. This is bounded by the depletion zones of the p-n junction between the first and second semiconductor regions on the one hand, and a further p-n junction between the first semiconductor region and the buried island region on the other hand. Particularly when switched off, the buried island region offers advantages, because that part of the p-n junction to the first semiconductor region which is located underneath the buried island region can absorb the majority of the reverse voltage.
In a further refinement, the buried island region is also electrically conductively connected to the second grid connection. The depletion zones of the p-n junctions between the two said p-conductive zones and the n-conductive first semiconductor region thus can be controlled jointly via the second grid connection.
Another advantageous embodiment of the electronic switching device envisages integration of the first and second semiconductor components to form a hybrid semiconductor structure. In this case, the electrode layer of the first anode connection and of the second cathode connection is used, in particular, as the connecting element between the two semiconductor components. Because both connections are short-circuited, they can be produced by a single electrode layer, which extends over the entire surface area of the hybrid semiconductor structure, between the first and second semiconductor components in the hybrid semiconductor structure. This configuration avoids mechanical stresses at the interface between the two semiconductor components, which occur in particular when the two semiconductor components are composed of different semiconductor materials and these different semiconductor materials come into contact with one another at the interface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electronic switching device having at least two semiconductor components, it is nevertheless not intended to be limited to the details shown, because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.